Photovoltaic arrays (PV arrays) are usually operated on the basis of a control method in which the maximum energy yield is obtained at the maximum power MPP (Maximum Power Point), for which reason the term MPP method is also used for this. In the case of partial shading of a photovoltaic array due to shading of one or more photovoltaic modules, the current-voltage characteristic (IU characteristic) of the photovoltaic array concerned changes, since the shaded photovoltaic modules represent a high ohmic resistance. Referred to here as the voltage is the DC link voltage U that can be controlled by means of an inverter. This results in a correspondingly low photovoltaic current and a strong voltage drop across the shaded photovoltaic modules. In order to avoid this, the current is made to pass by the shaded photovoltaic modules in each case with the aid of a bypass diode, which makes operation with a comparatively higher current possible. The alteration of the IU characteristic as a result of shading has the effect that the energy yield obtained according to the IU characteristic no longer has only one maximum, but a number of maxima (MPPs) as shown in FIG. 1. The control of the photovoltaic inverter on the basis of the MPP method is performed with what is known as an MPP tracker. In this case, the DC link voltage is set such that the power maximum MPP is obtained.
Without searching processes, it is possible that only one local maximum is found, which may have the consequence of considerable losses in yield. If, however, a searching process is started without there being any shading, losses in yield occur. However, it is intended to avoid this as far as possible.
WO 2006/081038A2 discloses a system and a method for determining a variable characteristic value within the operating range. FIG. 4 of WO 2006/081038A2 shows a searching process for use when there is partial shading of a photovoltaic array, a process in which a full search is carried out over the characteristic at regular intervals. This involves the inverter of the photovoltaic installation running through the entire operating range, and thereby using the current as the manipulated variable of the control.
According to DE 10 2010 036 966 A1, a search is carried out, depending on the probability that the operating point used at the given time is no longer at the global maximum. Here too, the inverter of the photovoltaic installation runs through the entire operating range. The search for the global maximum on the basis of a voltage difference between a past global maximum and a global maximum at the given time is used as the initialization of the search.
A further document (DHOPLE, BELL, DAVOUDI, CHAPMAN, DOMINGUEZ-GARCIA: A global maximum power point tracking method for PV module integrated converters. In: Energy Conversion Congress and Exposition (ECCE), 2012 IEEE, 2012, pp. 4762-4767) shows a device in which a search for the global maximum is only carried out when at least one bypass diode is forward biased.